--- /dev/null
+// Find CVE-2015-3193. Derived from
+// https://github.com/hannob/bignum-fuzz/blob/master/CVE-2015-3193-openssl-vs-gcrypt-modexp.c
+/* Fuzz-compare the OpenSSL function BN_mod_exp() and the libgcrypt function gcry_mpi_powm().
+ *
+ * To use this you should compile both libgcrypt and openssl with american fuzzy lop and then statically link everything together, e.g.:
+ * afl-clang-fast -o [output] [input] libgcrypt.a libcrypto.a -lgpg-error
+ *
+ * Input is a binary file, the first bytes will decide how the rest of the file will be split into three bignums.
+ *
+ * by Hanno Böck, license CC0 (public domain)
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <assert.h>
+#include <openssl/bn.h>
+#include <gcrypt.h>
+
+#define MAXBUF 1000000
+
+
+struct big_results {
+ char *name;
+ char *a;
+ char *b;
+ char *c;
+ char *exptmod;
+};
+
+void printres(struct big_results *res) {
+ printf("\n%s:\n", res->name);
+ printf("a: %s\n", res->a);
+ printf("b: %s\n", res->b);
+ printf("c: %s\n", res->c);
+ printf("b^c mod a: %s\n", res->exptmod);
+}
+
+void freeres(struct big_results *res) {
+ free(res->a);
+ free(res->b);
+ free(res->c);
+ free(res->exptmod);
+}
+
+
+char *gcrytostring(gcry_mpi_t in) {
+ char *a, *b;
+ size_t i;
+ size_t j=0;
+ gcry_mpi_aprint(GCRYMPI_FMT_HEX, (unsigned char**) &a, &i, in);
+ while(a[j]=='0' && j<(i-2)) j++;
+ if ((j%2)==1) j--;
+ if (strncmp(&a[j],"00",2)==0) j++;
+ b=(char*)malloc(i-j);
+ strcpy(b, &a[j]);
+ free(a);
+ return b;
+}
+
+/* test gcry functions from libgcrypt */
+void gcrytest(unsigned char* a_raw, int a_len, unsigned char* b_raw, int b_len, unsigned char* c_raw, int c_len, struct big_results *res) {
+ gcry_mpi_t a, b, c, res1, res2;
+
+ /* unknown leak here */
+ gcry_mpi_scan(&a, GCRYMPI_FMT_USG, a_raw, a_len, NULL);
+ res->a = gcrytostring(a);
+
+ gcry_mpi_scan(&b, GCRYMPI_FMT_USG, b_raw, b_len, NULL);
+ res->b = gcrytostring(b);
+
+ gcry_mpi_scan(&c, GCRYMPI_FMT_USG, c_raw, c_len, NULL);
+ res->c = gcrytostring(c);
+
+ res1=gcry_mpi_new(0);
+
+ gcry_mpi_powm(res1, b, c, a);
+ res->exptmod=gcrytostring(res1);
+
+ gcry_mpi_release(a);
+ gcry_mpi_release(b);
+ gcry_mpi_release(c);
+ gcry_mpi_release(res1);
+}
+
+/* test bn functions from openssl/libcrypto */
+void bntest(unsigned char* a_raw, int a_len, unsigned char* b_raw, int b_len, unsigned char* c_raw, int c_len, struct big_results *res) {
+ BN_CTX *bctx = BN_CTX_new();
+ BIGNUM *a = BN_new();
+ BIGNUM *b = BN_new();
+ BIGNUM *c = BN_new();
+ BIGNUM *res1 = BN_new();
+
+ BN_bin2bn(a_raw, a_len, a);
+ BN_bin2bn(b_raw, b_len, b);
+ BN_bin2bn(c_raw, c_len, c);
+
+ res->a = BN_bn2hex(a);
+ res->b = BN_bn2hex(b);
+ res->c = BN_bn2hex(c);
+
+ BN_mod_exp(res1, b, c, a, bctx);
+ res->exptmod = BN_bn2hex(res1);
+
+ BN_free(a);
+ BN_free(b);
+ BN_free(c);
+ BN_free(res1);
+ BN_CTX_free(bctx);
+}
+
+extern "C" int LLVMFuzzerTestOneInput(const unsigned char *Data, size_t Size) {
+ size_t len, l1, l2,l3;
+ unsigned int divi1, divi2;
+ unsigned char *a, *b, *c;
+ struct big_results openssl_results= {"openssl",0,0,0,0};
+ struct big_results gcrypt_results= {"libgcrypt",0,0,0,0};
+
+ len = Size;
+ if (len<5) return 0;
+
+ divi1=Data[0];
+ divi2=Data[1];
+ divi1++;divi2++;
+ l1 = (len-2)*divi1/256;
+ l2 = (len-2-l1)*divi2/256;
+ l3 = (len-2-l1-l2);
+ assert(l1+l2+l3==len-2);
+ //printf("div1 div2 %i %i\n", divi1, divi2);
+ //printf("len l1 l2 l3 %i %i %i %i\n", (int)len,(int)l1,(int)l2,(int)l3);
+ a=const_cast<unsigned char*>(Data)+2;
+ b=const_cast<unsigned char*>(Data)+2+l1;
+ c=const_cast<unsigned char*>(Data)+2+l1+l2;
+
+
+ bntest(a, l1, b, l2, c, l3, &openssl_results);
+ //printres(&openssl_results);
+ if ((strcmp(openssl_results.a,"0")==0) || (strcmp(openssl_results.c,"0")==0)) goto END;
+
+ gcrytest(a, l1, b, l2, c, l3, &gcrypt_results);
+ //printres(&gcrypt_results);
+
+ assert(strcmp(openssl_results.exptmod, gcrypt_results.exptmod)==0);
+
+END:
+ freeres(&openssl_results);
+ freeres(&gcrypt_results);
+ return 0;
+}
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